The invention relates to a circuit configuration for changing over the reception bands in high-frequency receivers, particularly in television and radio receivers. High-frequency receivers serve for selecting a desired frequency from a plurality of frequencies which are fed to the receiver, for instance via an antenna. The desired frequency is separated from the rest of the received frequencies, and the signal that is transmitted with that frequency is acquired by demodulation. The reception of different television or radio programs is possible by tuning the receiver to the frequency on which the respective program is broadcast.
The frequencies that are used for transmission purposes are divided into ranges, which are characterized as frequency bands. Receivers can typically be set to one frequency band or to several bands. The advantage of this is that a receiving unit need not be designed for all frequencies that are to be received; rather, subreceiver units that cover only one frequency band each can be provided. The tuning circuits in the subreceiver units are then optimized to the relatively small frequency spectrum of the respective band. Because one receiver unit is not provided to which all receivable frequencies are fed, but rather one of the subreceiver units must be selected according to the frequency that must be detected, a frequency band changeover is required. With the band changeover, that subreceiver unit is active which is allocated to the frequency band in which the desired frequency lies. The frequency that is detected by the respective subreceiver unit is then fed for further processing (e.g. demodulation) to a stage which is common to all subreceiver units.
The frequencies that are used in television transmission are divided into three bands. The lower VHF band (VHFI) encompasses the frequency spectrum from 45 to 126 MHz; the upper VHF band (VHFII) encompasses the frequencies from 133 to 407 MHz; and the UHF band encompasses the frequency interval from 415 to 860 MHz. For the VHF and UHF bands, electrically independent subreceiver units are usually used, which contain a respective mixer and oscillator. The band changeover is accomplished via specific control signals which switch on the desired mixer and oscillator.
Circuit configurations for band changeover in tuners are taught by EP 0 457 934 and EP 0 457 932. The circuit configuration according to EP 0457 932 is laid out for band changeover in tuners having at least two bands. For each band, an MOS tetrode is provided, which can be activated via a switching device. The MOS tetrode of one of the preliminary stages is active in that its source terminal is connected to the reference potential of the supply voltage via a switching device. The respective preliminary stage can thus be activated via the level of the potential at the source terminal of its tetrode. So that the tetrode is inactive when the source terminal does not lie at reference potential, a well defined potential must be set at a gate terminal and at the source terminal of the tetrode via voltage dividers. The disadvantage herein is that resistors with a low tolerance range must be used for the voltage divider. Another disadvantage is that the output signal at the drain terminal of the tetrode is dependent upon the dimensioning of the voltage divider.
It is accordingly an object of the invention to provide a circuit configuration for band changeover in high-frequency receivers which overcomes the above-mentioned disadvantageous of the prior art circuits of this general type, that is compatible with existing high-frequency receivers in terms of function and software and that has a reduced wiring and design outlay as compared to prior art circuits.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit configuration for changing between at least two bands in a high-frequency receiver, that includes a plurality of amplifier stages with a first amplifier stage associated with one of the at least two bands and a second amplifier stage associated with another one of the at least two bands. A switching mechanism is included for activating one of the plurality of amplifier stages at a time and thereby selecting one of the at least two bands. Each one of the plurality of amplifier stages includes a semiconductor switch element having two terminals connected in a main current path and having two control electrodes. One of the two terminals of the semiconductor switch element of each one of the plurality of amplifier stages is connected to a constant potential irregardless of which of the at least two bands is selected. The switching mechanism activates one of the plurality of amplifier stages at a time via one of the two control electrodes of the semiconductor switch element of the activated one of the plurality of amplifier stages.
The invention has the advantage that the resistors for the voltage divider at the source terminal are forgone. Accordingly, the dimensions of the layout are also reduced. Lastly, the space consumption of the inventive circuit configuration is reduced compared to conventional circuit configurations, which is particularly important in realizations in an integrated circuit.
In accordance with an added feature of the invention, the output signal of the semiconductor switch element is not dependent upon the wiring of the source terminal.
In accordance with an additional feature of the invention, the reference potential of the supply voltage is used as constant potential. Potential fluctuations can then be minimized without additional measures.
In accordance with another feature of the invention, one of the bands is divided into sub-bands by connecting a selection circuit. The coupling of an amplifier stage to the selection circuit is advantageous particularly for VHF band, which comprises lower and upper ranges.
In accordance with a concomitant feature of the invention, when the selection circuit that is provided for one range is connected to the control electrode of the semiconductor component for the other range in such a way that the selection circuit is only active when this semiconductor component is switched out. To this end, the selection circuit can be active in such a way that it is connected to the reference potential in a switchable fashion and is continuously at the supply voltage. When the selection circuit is connected to the reference potential, the control electrode of this semiconductor component is simultaneously brought approximately to the reference potential, whereupon it blocks. Simple npn transistors whose emitters are connected to the reference potential can be used for the circuit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for band changeover in high-frequency receivers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.